1. Field of the Invention
The present invention relates generally to polyamide-imide molding resins, and, more particularly, to phthalic anhydride modified polyamide-imide resins comprising amide-imide polymer obtained from reactants which comprise the reactant pair trimellitic anhydride chloride (hereafter "TMAC") and toluene diamine ("TDA"); or the reactant pair trimellitic anhydride ("TMA") and toluene diisocyanate ("TDI"). Accordingly, the invention is directed to a melt processible molding resin in which polymeric chains present in the resin comprise amide-imide repeating units having the following general formula: ##STR2## and wherein terminal amine groups of said polymeric chains are endcapped with phthalic anhydride moieties. Phthalic anhydride modified TMA/TDI or TMAC-TDA resins manufactured in accordance with the present invention exhibit improved melt-flow properties and greater thermal stability in the melt in comparison to the same resins without any phthalic anhydride endcapping, or in comparison to the same resins containing endcapping agents other than phthalic anhydride. The surprising improvement in both melt flow and melt thermal stability afforded by the present invention enhances the suitability of TMA-TDI or TMAC-TDA resins for use as engineering polymers in commercial molding applications.
2. Discussion of Background Art
Amide-imide resins are well known for their high temperature properties. In particular, the preparation of aromatic polyamide-imides from either trimellitic anhydride and aromatic diisocyanates, or from trimellitic anhydride chloride and aromatic diamines is disclosed in the literature. For example, R. Pigeon and P. Allard, in a published lecture entitled "Heat-Resistant and Flame-Resistant Fibers" (Die Angewandte Makromolekulare Chemie, Vol. 40/41, No. 600, pp 139-158, 1974) investigated the direct polycondensation reaction in polar solvent of trimellitic acid anhydride with different aromatic diisocyanates. Table 2 of this paper discloses polyamide-imides prepared from trimellitic acid anhydride and the 2,6 or 2,4 isomers of toluene diisocyanate.
Co-author P. Allard of the above-mentioned technical publication is also a named inventor of U.S. Pat. Nos. 3,929,691 and 3,903,058 and a co-inventor of Rochina et al. U.S. Pat. No. 3,717,696, all of which disclose polyamide-imide resins based on the polycondensation products of trimellitic acid anhydride and aromatic diisocyanates.
The Allard '691 patent discloses high molecular weight polyamide-imide copolymers derived from aromatic diisocyanates which contain two benzene nuclei (preferably ODI and MDI), an aromatic acid anhydride (preferably trimellitic acid anhydride), an aromatic or heterocyclic diacid (preferably isophthalic or terephthalic acids) and, optionally, a dianhydride (preferably pyromellitic dianhydride).
The above-mentioned Rochina et al. '696 patent discloses a process for producing polyamide-imide filaments by dry spinning a solution of polyamide-imide under specified conditions. The patent discloses for use as the spinning solution polyamide-imide solutions obtained by reacting in substantially stoichiometric proportions in a polar organic solvent at least one aromatic diisocyanate and an acid reactant containing at least an aromatic acid anhydride (preferably trimellitic acid anhydride) and optionally also at least one di-acid such as terephthalic or isophthalic acid. Toluene diisocyanate is disclosed in the patent as among the suitable diisocyanates for preparing the polyamide-imide solution. The patent also points out that the polyamide-imide can alternatively be prepared by reaction of a diamine with the chloride derivative of the acid anhydride reactant.
The Allard et al. '058 patent, like the patents discussed above, is directed to heat stable fibers based on polyamide-imide resins which are the reaction product of reactants comprising aromatic diisocyanates and aromatic acid anhydrides.
Serres et al. U.S. Pat. No. 3,839,529 discloses preparation of polyamide-imide filaments based on the reaction product of an acyl halide derivative of trimellitic acid anhydride which contains at least one acyl halide group in the 4-ring position, with aromatic primary diamines in polar organic solvents at temperatures below 150.degree. C. The patent discloses, as useful diamines for preparation of the polyamic acids, wholly or largely aromatic primary diamines, particularly aromatic primary diamines containing from 6 to about 10 carbon atoms or aromatic primary diamines composed of two divalent aromatic moieties of from 6 to about 10 carbon atoms, each moiety containing one primary amine group, with the moieties linked directly or through bridging groups such as --O--, --CH.sub.2 --, --CO--, --SO.sub.2 --, and --S--. Polyamic acids or polyamide-imides based on toluene diisocyanate or toluene diamine are not specifically disclosed.
The ability to manufacture melt-processible polyamide-imide resin based on toluene diamine or toluene diisocyanate is highly desirable due to the lower cost of these reactants as compared with other diamines such as, for example, 4,4' diisocyanato- (or diamino-) diphenylmethane and 4,4' diisocyanato- (or diamino-) diphenylether. In particular, the lower cost of toluene diisocyanate versus toluene diamine makes especially desirable the capability of producing a melt-processible TMA-TDI resin. Unfortunately, although the TMA/TDI and TMAC-TDA resins offer substantial cost savings over other amide-imide molding resins, there exists a need for improvement in the melt-flow and thermal melt stability of these resins in order to enhance the commercial suitability of the resins as engineering polymers for various high temperature molding applications. For example, the Tg of TMA-TDI resin having an I.V. of 0.5 is about 350.degree. C., while the decomposition temperature of the resin, as determined by thermogravimetric analysis is about 420.degree. C. Given this narrow window for melt processing, temperatures high enough to melt the resin will cause substantial polymer degradation and little if any appreciable melt flow in the resin. It is desired to widen the melt processing window, between the Tg of the polymer and the decomposition temperature, in order to afford greater manufacturing latitude in molding operations. In addition, if the decomposition temperature of the polymer can be raised, the overall thermal stability of products manufactured from the TMA-TDI or TMAC-TDA can be enhanced.
The modification of certain amide-imide resins, which are chemically distinct from the amide-imide resins dealt with in the present invention, has been disclosed in the art. In this regard, Hanson U.S. Pat. Nos. 4,448,925 and 4,722,992 disclose injection moldable amide-imide phthalamide polymers comprising phthalic anhydride moieties. In particular, Hanson '925 discloses that the incorporation of from about 1 to about 10% phthalic anhydride in the polyamide-imide phthamide polymers greatly improves the flow property of these resins. In addition to phthalic anhydride, the '992 patent discloses incorporation of 1 to 10% aniline or TMA for improved flow properties. Neither of the Hanson patents discloses or suggests the incorporation of phthalic anhydride in amide-imide resins based on TMA-TDI or TMAC-TDA.
A general object of the present invention is to improve the melt processibility of amide-imide molding resins based on TMA-TDI or TMAC-TDA. Other objects will become apparent hereinafter to those skilled in the art.